1. Field of the Invention
The present invention relates to a semiconductor structure provided with quantum boxes or quantum wires therein and a method of producing the same.
The semiconductor structure according to the present invention is suitable for an optical semiconductor device having excellent performance and a semiconductor device operating at an ultra-high speed.
2. Description of the Related Art
In order to improve the performance of an optical apparatus and an electronic apparatus, such as an electronic computer, there is a demand for optical semiconductor devices and electrical semiconductor devices that have higher performance. A semiconductor quantum box or wire utilizing the quantum mechanical properties of electrons can provide a semiconductor device structure meeting the demand.
Such a semiconductor quantum box or wire would afford a lower threshold current of a semiconductor laser, improved performance of an optical nonlinear material, and larger integration, higher speed and more functional operation of a semiconductor integrated circuit. However, it is very difficult to produce a semiconductor structure including a plurality of quantum boxes or quantum wires, both easily and reliably.
Referring to FIG. 1, quantum boxes are formed on a semiconductor substrate 1. In this case, a semiconductor clad layer 2, a semiconductor thin (well) layer 3 for the quantum boxes, and a semiconductor clad layer 4 are epitaxially grown on the semiconductor substrate 1. Fine pattern masks 5 are formed by applying a resist layer on the clad layer 4, exposing it with a fine lithography (e.g., an electron beam lithography process and developing it. Then the layers 2, 3 and 4 are selectively etched by a suitable dry etching method to form the semiconductor quantum boxes 3, each sandwiched between respective clad layers 2 and 4. The quantum boxes 3 can have a diameter of from 10 to 20 nm, which is capable of three-dimensionally confining electrons to create a quantum mechanical effect.
Referring to FIG. 2, quantum boxes are formed on a semiconductor substrate 11 by using the crystallinity of a single crystal semiconductor. In this case, a SiO.sub.2 layer 12 is formed on a GaAs substrate 11, and then is selectively etched to form triangle openings by a conventional lithography (e.g., an electron beam lithography) and a dry etching method. AlGaAs portions 13 are epitaxially grown on the GaAs substrate 11 through the openings by using an Al source, a Ga source and an As source, and then the Al source supply is stopped to successively epitaxially grow GaAs portions 14 on the AlGaAs portions 13, respectively, with the result that each of triangular pyramids consists of a truncated triangular pyramid base portion 13 and a triangular pyramid top portion (quantum box) 14. One of the base sides of the GaAs pyramid quantum box can be about 10 nm or less, which is capable of three-dimensionally confining electrons in the box to create a quantum mechanical effect.
Although the quantum boxes are formed simultaneously, the sizes of the boxes are different from each other (i.e., are not uniform) because of a low accuracy in the patterning processes (exposure, development and etching tolerances). For example, when the sizes of the triangle openings in the SiO.sub.2 layer are not the same within the tolerance of lithography, the grown triangular pyramids have different heights, and thus the sizes of the triangular pyramid top portions (quantum boxes) are not the same among the boxes. Each of the quantum boxes has a peak density state .rho.(E) of electron energy level. Where the dimensions of the quantum boxes are almost the same, the respective energy levels of the quantum boxes have substantially the same value with the result that a peak is obtained in the distribution of the density of states, as shown in FIG. 3. On the other hand, when the dimensions of the boxes are not the same, the respective energy levels of the quantum boxes are distributed, as shown in FIG. 3, with the result that quantum mechanical effects are not effectively attained.
In the case of a quantum wire semiconductor structure, similar problems are caused by size fluctuations, or differences, in lithography.